1 1/2 Socket to Mm | The Exact Conversion

For the full breakdown, see our best 1 1/2 Socket guide.

If you’re staring at an SAE fastener and a metric socket set, the difference between 38.1 mm and a standard 38 mm socket rarely matters in practice. The conversion itself, however, is exact: multiply 1.5 inches by 25.4 to get 38.1 mm. That 0.1 mm gap between the true measurement and the closest socket you can actually buy is where most of the confusion starts.

Converting 1 1/2 Inches to Millimeters

The math behind this conversion is fixed and simple. One inch equals 25.4 millimeters, so a 1 1/2 inch socket converts by multiplying 1.5 by 25.4.

• 1.5 × 25.4 = 38.1 mm

When you shop for a metric socket, manufacturers don’t make fractional sizes like 38.1 mm — they make whole numbers. The closest standard metric socket is 38 mm, which sits just 0.1 mm smaller than the exact conversion. Tekton’s conversion charts list 1 1/2 inch as 38.10 mm and pair it with 38 mm as the closest socket size, and Misumi’s socket chart confirms the reverse match.

Is 38 mm or 39 mm the Right Socket?

For the SAE-to-metric match, 38 mm is the correct choice over 39 mm.

Some conversion tables list 39 mm as a “closest” option, especially for worn or loose fasteners, but that’s a practical judgment call, not an arithmetic one. Teng Tools’ standard-to-metric chart treats 38 mm as the direct equivalent of 1 1/2 inch for the same reason.

Common Mistakes With 1 1/2 Inch Sockets

Two errors trip up most people working with this size, and both come from mixing up related but different things.

• Treating 39 mm as exact. It’s not. The exact conversion is 38.1 mm; 39 mm is only occasionally useful for loose fits.
• Confusing drive size with socket size. A 1/2 inch drive refers to the square anvil that connects to the ratchet, not the opening that grips the fastener. A 1/2 inch drive ratchet can absolutely carry a 1 1/2 inch socket — the two numbers describe different parts entirely.

That last mix-up is common enough that it’s worth stressing: the drive size never tells you what fastener a socket fits. You need to read the socket’s opening size, not the drive end.

Checking Fit and Staying Safe Under Torque

Even with the right size picked, always test the socket on the actual fastener before applying serious force. A socket that seats fully and grips without rocking is correct; one that slips or sits loose will round off the bolt head under torque. Metric and SAE sockets aren’t universally interchangeable even when numbers look close, and the 0.1 mm difference here is exactly the kind of slack that matters on a stubborn fastener.

If you’re working with this socket size regularly and want to see tested options that fit the job, our roundup of the best 1 1/2 inch sockets covers the top picks in detail.

The practical takeaway: for a 1 1/2 inch fastener, reach for a 38 mm metric socket first, confirm it seats fully, and torque with confidence.